Clad cermet body



Aug- 1, 1951 J. P. DENNY ETAL 2,994,124

CLAD CERMET BODY Filed 00's. 3, 1955 INVENT R JUA/N f? E/VA/l/o 2,994,124 Y CLAD CERMET BODY John P. penny, Schenectady, N.Y., and Edwin D. Sayre and William F. Zimmerman, Cincinnati, Ohio, assignors to General Electric Company, a corporation of New York Filed Oct. 3, 1955, Ser. No. 538,172

1 Claim. (Cl. 29-183.5)

This invention relates to composite cermet bodies and, more particularly, to claddings for cermet bodies which improve their resistance to impact and thermal shock at ambient and elevated temperatures and a method of applying such claddings.

One of the current problems in the metallurgical field is the development of materials which will operate satisfactorily at extremely high temperatures such as are encountered in a heat engine, for example. It has been well known that the operating eliiciency of a heat engine, suc-h as a gas turbine, may be improved by raising the operating temperature to as high a value as possible. It necessarily follows that heat engines are operated at as high a temperature as presently available materials are able to withstand satisfactorily. High temperature metal alloys which are normally adequate and innately strong and corrosion resistant at temperatures up to about 1500u F., lose their strength at increased temperatures. In the temperature ran-ge between 1500 F. and 1800 F., some of the precipitation hardening alloyslose their strength due to return to solution of the precipitated phases.

One current solution to the Search lfor satisfactory high temperature materials is the use of a composite body known as a cermet material. A cermet is a metal-ceramic composite and an intermediate between metals `and nonmetals. Cermets generally comprise an inorganic compound and a metallic constituent metallurgically bonded by mechanical or chemical means. However, the properties of the resulting product usually diiferfrom those of the parent constituents.

Cermet bodies have been found to have good strength characteristics at temperatures around 2000 F. or above dep-ending upon the metal constituent whose melting or softening point -generally limits operating temperatures. in the use of cermets,problems which have been solved by industry over the past several years include improvement of oxidation resistance, thermal-shock resistance and stress rupture strength. Satisfactory manufacturing methods have also been developed'for intricate shapes such as those of a turbine bucket or nozzle diaphragm partition which `are parts used on a gas turbine engine.

A typical cermet may, for example, have a principal constituent of a carbide, boride, silicide or oxide structure combined with a refractorymetal such as nickel, cobalt, chromium, columbium, tantalum, titanium, zirconium, tungsten, molybdenum or vanadium and their alloys as an impregnating phase. By varying the combination of these constituents a cermet having the desirable properties enumerated above can be fabricated. Unfortunately, however, all ofthe various combinations that can be made have the undesirable characteristic of having a very low resistance to impact. This deficiency-is particularly undesirable in applications where the part may be subjected to extremely high fluid 'velocities such as in a gas turbine engine. In such applications there are small, hard, sharp, foreign particles, such as sand, ow-

ing through the engine which strike against the cermet material. Due to the extremely high fluid rvelocities as Ufound in gas turbine applications the particles cause imj pact failures of the cermet'articles. The eifect of `a blow by such foreign particles is the setting'up, at the point of .imp-a?, f tremendously high laalizd Stresses .which intatti Patented Aug. l, 1961 tend to crack or chip the cermet material. A crack or chip thus produced generally results in the disintegration of the part formed from the cermet. By varying the combination of the above mentioned constituents the impact resistance of the cermet material can be improved only to a limited degree.

A typical cermet base material for gas turbine cornponents, taking overall properties into consideration, has titanium carbide as the major phase. It has low specic gravity, high hardness, high stress rupture strength and good thermal shock resistance. However, at elevated temperatures, titanium carbide forms on its surface an extremely stable oxide film which is diicult to reduce in the very purest hydrogen atmosphere. This surface lm will prohibit the joining of a protective coating to t-he metallic bonded titanium carbide. These fdrns can be removed by diamond grinding but they will form again at elevated temperatures even in protective and/or reducing atmospheres.

We have found, experimentally, that the impact resistance of a cermet body may be increased by a factor of as much as 5 or more by cladding the cermet With a satisfactory high temperature and oxidation resistant sheet metal alloy to form a composite body. The sheet metal is bonded to the composite cermet body. One of the difficulties encountered, however, in attaching the sheet metal to the cermet was the aforementioned formation of extremely stable and difficult to reduce oxides which make it p-ractically impossible to obtain a satisfactory fusion bond between the sheet metal material and the composite cermet.

One of the objects of this invention is to attach a high temperature oxidation resistant sheet metal material to the surface of a composite body or cermet article to increase its impact resistance.

Another object of this invention is to provide a satisfactory method of joining the sheet metal material to the composite cermet body despite the formation of extremely stable and dicult to 4reduce oxides on the surface of the cermet body.

Briey stated, in accordance with one aspect of our invention, we provide a composite cermet body having a high temperature and oxidation resistant sheet metal cladding in order to substantially increase the impact lresistance of the article or body. We attach the sheet metal by first spraying the body with a nickel slurry `and sintering to obtain a nickel coating of approximately .004" over the body. We form the sheet metal to coincide with the shape of the body, hold it in intimate contact with the body and bond the sheet metal in a reducing atmosphere for suicient time to obtain a -good bond of the sheet metal to the nickel coating.

' Theinvention hereinafter described is applicable to any regularly or irregularly shaped cermet body that is to have its impact resistance increased at ambient or elevated temperatures. For purposes o-f illustration, however, the invention will be `described in connection with the cladding of a gas turbine bucket since this is a rather typical application for this invention. It is to be understood, however, that the cladding as hereinafter described is applicable to any cermet body whether it is a fiat sheet or an irregularly shaped object, the latter being illustrated bv a turbine bucket as hereinafter set forth.

Oui invention will be better understood from the following description taken in connection lwith the accompanying draw-ing and its scope will be pointed out in the appended claims.

In the drawing, FIGURE l isa perspective View of a turbine bucket; and FIGURES 2, 3, 4 and 5 aretypical cross-sections of a clad turbine bucket showing various cladding systems that may be employedrwherein 'the thickness of the cladding is greatly exaggerated for purposes of illustration.

Referring to the drawing, FIGURE l shows a typical turbine bucket. The turbine bucket as generally indicated at 10 comprises a base or yroot portion 1'2. and an airfoil or blade portion 14. The turbine bucket 10 may be made from the various combinations of constituents to form a composite cerrnet bucket as previously explained. The formed composite cerrnet bucket comprises the inner portion 16 in FIGURE 2. As previously mentioned, a more desirable composition for turbine buckets is a titanium carbide cerrnet and vthe invention as hereinafter described will discuss a bucket lwith titanium carbide as the major phase although it is to be understood that other carbides, borides, silicides or oxides may be yused as the major phase in practicing the invention. Titanium carbide is used for purposes of illustration merely because it has been found to be a more desirable material of those now available .as turbine buckets. A typical titanium carbide cermet which may .be used has a composition, by weight, of 62% titanium carbide, 8% titanium-tantalum-columbium carbide mixed, 25% nickel .and molybdenum. A finished cerrnet bucket having the aforementioned composition is a commercially available item and the starting point for the present invention would be the procurement of a turbine bucket having such or any similar composition. Cladding the titanium carbide cerrnet to improve its impact resistance may then be carried out as subsequently described herein.

Due to the fact that highly stable and dicult to reduce oxides form on the surface of a titanium carbide cermet, it is not possible to obtain, directly, a satisfactory bond between the cladding material and the titanium carbide. We found, therefore, that it was necessary to provide an intermediate coating Which may be applied by following the procedure to be hereinafter described.

'I'he outer surface of the inner portion 16 of a cerrnet turbine bucket is iirst completely cleaned using any well known cleaning technique such as light sand blasting or degreasing. Such cleaning techniques are so well known in the art that no detailed procedure is thought necessary.

In order to overcome the problems raised by the formation of oxides on the titanium carbide, we apply a nickel coating 18 to the surface of the titanium carbide inner portion 16. This is done by first spraying the surface of the inner portion 16 with a slurry of nickel powders. Good results have been obtained by using a nickel slurry having a composition of 7 cc. of minus 325 mesh nickel powder; 25 cc. of benzene or toluene, 25 cc. of acetone and 1/2 cc. of a solution of 40% acrylic resin in toluene. The nickel slurry may be applied by any type of suitable spraying equipment. The slurry is sprayed to leave a uniform thickness of approximately .004" after the sintering step which follows the spraying step. 'It has been found that the proper thickness for the nickel slurry may be obtained by spraying the above mentioned composition uniformly on the titanium carbide such that the weight of the bucket is increased 0.5 gram per square inch. This will result in a nickel coating of approximately .004 after sintering. It is to be noted that the thickness of approximately .004 is critical and the coating should be held as nearly to thatV value as possible. Experiments have shown that if the thickness is substantially less than .004" the nickel tends to diffuse completely into the cerrnet in some areas of the airfoil l14; if the coating is appreciably thicker than .004, there is a tendency for the coating to form shrinkage cracks and peel during sintering. Therefore the thickness of approximatelyl .004 should be maintained as closely as possible. After spraying on the nickel slurry to the proper thickness, the bucket is placed in a vacuum furnace at an absolute pressure of approximately 10-15 microns for sintering. The temperature is gradually brought up, for a period of one hour, for example, to a maximum of 2280a F. When the temperature reaches 2280 F. the furnace is abruptly shut down. The bucket is left in the furnace until it has completely cooled to room temperature. Upon cooling the bucket is removed and upon examination `it has a nickel coating 18 of approximately .004 thickness sintercd to lthe titanium carbide inner portion 16.

The nickel coated bucket is next prepared forthe bonding of the outer sheet metal alloy material to the bucket. The outer surface of the nickel coating 18 is first polished with a iine grain emery cloth to remove surface oxides and to allow better bonding.

After polishing the nickel coating, a brazing alloy may be applied over the nickel coating 18 to promote formation of a bond when fusingthe outer cladding 20 to the nickel coating 18. It is to be understood that the use of a brazing alloy is not an essential step in fusing the outer cladding 20 to the nickel coating l18. The brazing alloy may be helpful in some instances to expedite the formation of the diffusion bond. Any well known nickel base brazing alloy suitable for this application may be used such as one which has a nominal composition of 85% nickel, 2.3% boron, 3.5% silicon with the balance iron.

The outer cladding 20 is then prepared for bonding to the nickel coating 18. The outer cladding 20 is cut and shaped from a heat and oxidation resistant sheet metal alloy in order to i'it over the blade or airfoil portion 14 of the bucket 10. Any of a great number of well known high :temperature and oxidation resistant sheet metal alloys may be used for this purpose. An example of a number of compositions which are satisfactory are 20% chromium, nickel; 16% chromium, 80% nickel, 3.75% aluminum and 0.45% titanium; 19.5% chromium, 10% nickel, 51% cobalt, 14.5% tungsten and 2.5% iron; and 25% chromium, 20% nickel and 55% iron. It has been found -that a sheet material with a thicknses of .006l gives very satisfactory results. It is to be understood, of course, that other thicknesses may be used to obtain the desired -results of this invent-ion and the thickness of .006 is given merely by way of illustration. The sheet material is formed or cold worked so that it will lie intimately in contact with the contours of the airfoil 14 of .the vturbine bucket 10. Shaping of the sheet material may be .done by any well known mechanical means such as vmechanical `rollers or rubberstatic dies.

The sheet material for cladding the bucket is next held against 4the bucket having a nickel coating 18. Suitable dies are provided to hold the outer cladding 20 in intimate contact with the nickel coating 18 and the assembled bucket, outer cladding and dies are placed in a furnace having a hydrogen reducing atmosphere. The outer cladding 20 is bonded to the nickel coating 18 by bringing the temperature of the furnace to 2250-2300 F. in about l5-20 minutes and holding that temperature for approximately ve minutes. 'I'he nished bucket is furnace cooled inthe hydrogen atmosphere until it again returns to room temperature. Upon removal from the furnace, the bucket has the cladding 20 bonded thereto and this product has substantial increased resistance to impact as compared to the cerrnet bucket without any cladding.

Still further resistance to impact may be obtained by l the use'of intermediate sheet metal layers of a corrugated `rial although they are of the same types. The intermediate layers may 'be joined to the immediately preceding layer, inf all cases, by the same method as used in joining the outer cladding 20 to the nickel coating 18 of the FIGURE 2 configuration.

FIGURE 3 shows a corrugated intermediate sheet metal portion 22 bonded to the nickel coating 18 and an outer smooth heat and oxidation resistance sheet 24 bonded to the intermediate corrugated portion 22.

The modication of FIGURE 4 is basically -the cladding system of FIGURE 2 having a corrugated intermediate sheet metal portion 22 bonded to the cladding 20 and an outer smooth sheet 24 bonded to the intermediate sheet metal corrugated portion 22.

The system of FIGURE 5 is the same as that of FIG- URE 3 with the additional intermediate corrugated layer 26 bonded to the intermediate smooth layer 24 and an outer smooth layer 28 bonded to the corrugated layer 26.

While particular embodiments of the invention have been illustrated and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the inrvention and it is intended to cover in the appended claims all such changes and modcations that come within the true spirit and scope of the invention.

What We claim -as new and desire to secure by Letters Patent of the United States is:

A composite article manufactured from a titanium carbide cermet consisting of by weight about about 62% titanium carbide, 8% titanium-tantalum-columbium carbide mixture, 25% nckel and 5% molybdenum, said article having a rst coating of about 0.004 in thickness of sintered nickel powder bonded with said cermet and a second coating of a metal selected from the group consisting of iron, chromium, nickel, cobalt and their alloys bonded with said rst coating.

References Cited in the le of this patent UNITED STATES PATENTS 1,972,307 Loetscher Sept. 4, 41934 2,020,477 Scott NOV. 12, 1935 2,048,276 Marlies July 21, 19136 2,117,085 Ensminger May 10, 19138 2,169,090 Dawihl Aug. 8, 1939 2,289,614 Wesley July 14, '1942 2,377,177 Peumer May 29, `1945 2,431,660 Gaudenzi NOV. 25, 1947 2,520,373 Price Aug. 2-9, 1950 2,681,876 De Santis et al. June 22, 1954 2,763,919 Kempe Sept. 25, 1956 2,798,577 LaFOrge July 9, 1957 FOREIGN PATENTS 213,483 Switzerland Feb. 15, 1941 602,530Y Great Britain May 28, 1948 

